12 Answers
12

When you declare a reference variable (i.e. an object) you are really creating a pointer to an object. Consider the following code where you declare a variable of primitive type int:

int x;
x = 10;

In this example, the variable x is an int and Java will initialize it to 0 for you. When you assign it to 10 in the second line your value 10 is written into the memory location pointed to by x.

But, when you try to declare a reference type something different happens. Take the following code:

Integer num;
num = new Integer(10);

The first line declares a variable named num, but, it does not contain a primitive value. Instead, it contains a pointer (because the type is Integer which is a reference type). Since you did not say as yet what to point to Java sets it to null, meaning "I am pointing at nothing".

In the second line, the new keyword is used to instantiate (or create) an object of type Integer and the pointer variable num is assigned this object. You can now reference the object using the dereferencing operator . (a dot).

The Exception that you asked about occurs when you declare a variable but did not create an object. If you attempt to dereference num BEFORE creating the object you get a NullPointerException. In the most trivial cases, the compiler will catch the problem and let you know that "num may not have been initialized" but sometimes you write code that does not directly create the object.

For instance, you may have a method as follows:

public void doSomething(SomeObject obj) {
//do something to obj
}

In which case you are not creating the object obj, rather assuming that it was created before the doSomething method was called. Unfortunately, it is possible to call the method like this:

doSomething(null);

In which case obj is null. If the method is intended to do something to the passed-in object, it is appropriate to throw the NullPointerException because it's a programmer error and the programmer will need that information for debugging purposes.

Alternatively, there may be cases where the purpose of the method is not solely to operate on the passed in object, and therefore a null parameter may be acceptable. In this case, you would need to check for a null parameter and behave differently. You should also explain this in the documentation. For example, doSomething could be written as:

"The best way to avoid this type of exception is to always check for null when you did not create the object yourself." If the caller passes null, but null is not a valid argument for the method, then it's correct to throw the exception back at the caller because it's the caller's fault. Silently ignoring invalid input and doing nothing in the method is extremely poor advice because it hides the problem.
– BoannJul 29 '14 at 13:32

83

I would add a remark about this post explaining that even assignments to primitives can cause NPEs when using autoboxing: int a=b can throw an NPE if b is an Integer. There are cases where this is confusing to debug.
– Simon FischerSep 26 '14 at 19:45

45

Is it possible to capture NPE thrown by a webapp from the web browser?like will it show in the view page source from the web browser..
– SidApr 13 '15 at 14:51

62

Yes check if the object equals null before you invoke a method on it or try to access a variable it might have. Some times structuring your code can help avoid null pointer exception. eg when checking an input string with a constant string you should start with the constant string like here: if ("SomeString".equals(inputString)) {} //even if inputString is null no exception is thrown. So there are a bunch of things that you can do to try to be safe.
– RoseNov 11 '15 at 4:39

64

An additional way of avoiding NullPointerException problems in your code is to use @Nullable and @NotNull annotations. The following answer has more information on this. Although this answer is specificially about the IntelliJ IDE, it is also applicable to other tools as is apparanet from teh comments. (BTW I am not allowed to edit this answer directly, perhaps the author can add it?)
– Arjan MelsJan 3 '16 at 18:17

NullPointerExceptions are exceptions that occur when you try to use a reference that points to no location in memory (null) as though it were referencing an object. Calling a method on a null reference or trying to access a field of a null reference will trigger a NullPointerException. These are the most common, but other ways are listed on the NullPointerException javadoc page.

Probably the quickest example code I could come up with to illustrate a NullPointerException would be:

On the first line inside main, I'm explicitly setting the Object reference obj equal to null. This means I have a reference, but it isn't pointing to any object. After that, I try to treat the reference as though it points to an object by calling a method on it. This results in a NullPointerException because there is no code to execute in the location that the reference is pointing.

(This is a technicality, but I think it bears mentioning: A reference that points to null isn't the same as a C pointer that points to an invalid memory location. A null pointer is literally not pointing anywhere, which is subtly different than pointing to a location that happens to be invalid.)

I understood everything you wrote there, but only because I've been coding for a while and know what a 'pointer' and a 'reference' are (and what null is, for that matter). When I try to dive right into explanations like that, my students look at me crosseyed, because there's not enough background.
– mmrFeb 20 '09 at 4:06

26

@mmr: Thanks for the feedback, you make a valid point. It's difficult on the internet to really judge where someone is at, and at what level it's safe to start an explanation. I'll try revising this again.
– Bill the LizardFeb 20 '09 at 4:32

17

A more common way to get a NullPointerException in practice would be forgetting to explicitly initialize a member variable to something other than null before using it, like this. With local variables, the compiler would catch this error, but in this case it doesn't. Maybe that would make a useful addition to your answer?
– Ilmari KaronenJan 18 '14 at 9:28

The first step is identifying exactly which values are causing the exception. For this, we need to do some debugging. It's important to learn to read a stacktrace. This will show you where the exception was thrown:

Exception in thread "main" java.lang.NullPointerException
at Printer.printString(Printer.java:13)
at Printer.print(Printer.java:9)
at Printer.main(Printer.java:19)

Here, we see that the exception is thrown on line 13 (in the printString method). Look at the line and check which values are null by
adding logging statements or using a debugger. We find out that s is null, and calling the length method on it throws the exception. We can see that the program stops throwing the exception when s.length() is removed from the method.

Trace where these values come from

Next check where this value comes from. By following the callers of the method, we see that s is passed in with printString(name) in the print() method, and this.name is null.

Trace where these values should be set

Where is this.name set? In the setName(String) method. With some more debugging, we can see that this method isn't called at all. If the method was called, make sure to check the order that these methods are called, and the set method isn't called after the print method.

This is enough to give us a solution: add a call to printer.setName() before calling printer.print().

Other fixes

The variable can have a default value (and setName can prevent it being set to null):

private String name = "";

Either the print or printString method can check for null, for example:

I still can't find the problem

If you tried to debug the problem and still don't have a solution, you can post a question for more help, but make sure to include what you've tried so far. At a minimum, include the stacktrace in the question, and mark the important line numbers in the code. Also, try simplifying the code first (see SSCCE).

+1 Good to have an example that includes going through the stacktrace; it's important to show why reading it is important for debugging NPE. (and why we almost always look for a stacktrace when someone posts a question about an error)
– Dennis MengJun 8 '14 at 1:30

14

You mentioned debugging...How does that work? I have been researching the topic for a while now, but can find nothing. I'm sure an amazing teacher like you can teach it to me in a second! Thanks so much! :-)
– Ruchir BaroniaDec 10 '15 at 7:07

13

@RuchirBaronia A debugger allows you to step through a program line by line to see which methods are called and how variables are changed. IDEs should have some tools to do this. See vogella.com/tutorials/EclipseDebugging/article.html for example.
– fgbDec 10 '15 at 10:22

13

@RuchirBaronia You set breakpoints on the methods around any NullPointerExceptions as seen in the stacktrace, and check the values of variables against what you expect them to be. If you know a variable is null when it shouldn't be, then you can set breakpoints around any code that changes the value. There are also conditional breakpoints you can use which will tell you when a value changes.
– fgbDec 10 '15 at 11:33

5

Setting String objects to an empty string as their default value is considered to be a poor practice.
– TinyFeb 27 '16 at 2:05

Question: What causes a NullPointerException (NPE)?

As you should know, Java types are divided into primitive types (boolean, int, etc.) and reference types. Reference types in Java allow you to use the special value null which is the Java way of saying "no object".

A NullPointerException is thrown at runtime whenever your program attempts to use a null as if it was a real reference. For example, if you write this:

Question: How do I read the NPE stacktrace?

First observation: the compilation succeeds! The problem in the program is NOT a compilation error. It is a runtime error. (Some IDEs may warn your program will always throw an exception ... but the standard javac compiler doesn't.)

Second observation: when I run the program, it outputs two lines of "gobbledy-gook". WRONG!! That's not gobbledy-gook. It is a stacktrace ... and it provides vital information that will help you track down the error in your code, if you take the time to read it carefully.

So let's look at what it says:

Exception in thread "main" java.lang.NullPointerException

The first line of the stack trace tells you a number of things:

It tells you the name of the Java thread in which the exception was thrown. For a simple program with one thread (like this one), it will be "main". Let's move on ...

It tells you the full name of the exception that was thrown; i.e. java.lang.NullPointerException.

If the exception has an associated error message, that will be output after the exception name. NullPointerException is unusual in this respect, because it rarely has an error message.

The second line is the most important one in diagnosing an NPE.

at Test.main(Test.java:4)

This tells us a number of things:

"at Test.main" says that we were in the main method of the Test class.

"Test.java:4" gives the source filename of the class, AND it tells us that the statement where this occurred is in line 4 of the file.

If you count the lines in the file above, line 4 is the one that I labeled with the "HERE" comment.

Note that in a more complicated example, there will be lots of lines in the NPE stack trace. But you can be sure that the second line (the first "at" line) will tell you where the NPE was thrown1.

In short the stack trace will tell us unambiguously which statement of the program has thrown the NPE.

1 - Not quite true. There are things called nested exceptions...

Question: How do I track down the cause of the NPE exception in my code?

This is the hard part. The short answer is to apply logical inference to the evidence provided by the stack trace, the source code and the relevant API documentation.

Let's illustrate with the simple example (above) first. We start by looking at the line that the stack trace has told us is where the NPE happened:

int length = foo.length(); // HERE

How can that throw an NPE?

In fact there is only one way: it can only happen if foo has the value null. We then try to run the length() method on null and .... BANG!

But (I hear you say) what if the NPE was thrown inside the length() method call?

Well, if that happened, the stack trace would look different. The first "at" line would say that the exception was thrown in some line in the java.lang.String class, and line 4 of Test.java would be the second "at" line.

So where did that null come from? In this case it is obvious, and it is obvious what we need to do to fix it. (Assign a non-null value to foo.)

OK, so let's try a slightly more tricky example. This will require some logical deduction.

Looking at the first line, how could that throw an NPE? There are two ways:

If the value of bar is null then bar[pos] will throw an NPE.

If the value of bar[pos] is null then calling length() on it will throw an NPE.

Next, we need to figure out which of those scenarios explains what is actually happening. We will start by exploring the first one:

Where does bar come from? It is a parameter to the test method call, and if we look at how test was called, we can see that it comes from the foo static variable. In addition, we can see clearly that we initialized foo to a non-null value. That is sufficient to tentatively dismiss this explanation. (In theory, something else could changefoo to null ... but that is not happening here.)

So what about our second scenario? Well, we can see that pos is 1, so that means that foo[1] must be null. Is that possible?

Indeed it is! And that is the problem. When we initialize like this:

private static String[] foo = new String[2];

we allocate a String[] with two elements that are initialized to null. After that, we have not changed the contents of foo ... so foo[1] will still be null.

A NULL pointer is one that points to nowhere. When you dereference a pointer p, you say "give me the data at the location stored in "p". When p is a null pointer, the location stored in p is nowhere, you're saying "give me the data at the location 'nowhere'". Obviously, it can't do this, so it throws a NULL pointer exception.

"A NULL pointer is one that points to nowhere" I disagree. Null pointers don't point to nowhere, they point to null values.
– TheRealChx101Sep 8 '18 at 22:46

1

@TheRealChx101 A null pointer and a pointer to a null value are different things - a null pointer does not point to a null value. Suppose you have a pointer to a pointer: pointer A points to pointer B, and pointer B is null. In this case, pointer A points to a null value, and pointer B is a null pointer.
– MrZebraSep 10 '18 at 7:43

It is worth mentioning that some IDEs (e.g. Eclipse) offer automatic nullity analisys based on customizable annotations (e.g. @Nullable as listed above) and warn about potential errors. It is also possible to infer and generate such annotations (e.g. IntelliJ can do that) based on existing code structure.
– Jan ChimiakMar 5 '16 at 10:34

3

First thing should do is before using a nullable object, you should check whether is it null, using if (obj==null).If it is null then you should write code to handle that also.
– Lakmal VithanageFeb 17 '17 at 6:52

3

IMO, it is preferable to avoid returning null objects in methods when possible and use annotation when null input parameters are not allowed in order to, by contract, reduce the amount of ´if (obj==null)´ in the code and improve the code readability.
– L. G.Feb 27 '17 at 9:43

While this is a nice example, may I ask what it adds to the question that isn't already covered by all the other answers?
– MysticialSep 24 '13 at 6:20

13

It is simply inappropriate to use the word "uninitialized" here. The example you shown is in fact "initialized", and it is initialized with null. For uninitialized variables, compiler will complain to you.
– Adrian ShumSep 24 '13 at 6:32

2

An NPE may be an indicator that you are using an uninitialized field. It may be an indicator that you are doing other things. Oversimplifying to a single cause like this does not help someone solve NPE problems ... if the actual cause is not this one.
– Stephen CMay 27 '15 at 13:35

Declaring a primitive variable, such as an int, in Java does not create an Object.
– Michael KrauseNov 15 '16 at 21:44

3

A NullPointerException often occurs when calling method of an instance.For example, if you declare a reference but does not make it point to any instance, NullPointerException will happen when you call its method. such as: YourClass ref = null; // or ref = anotherRef; // but anotherRef has not pointed any instance ref.someMethod(); // it will throw NullPointerException. Generally fix it in this way: Before the method is called, determine whether the reference is null. such as: if (yourRef != null) { yourRef.someMethod(); }
– sunhangJul 28 '17 at 10:25

In Java all the variables you declare are actually "references" to the objects (or primitives) and not the objects themselves.

When you attempt to execute one object method, the reference asks the living object to execute that method. But if the reference is referencing NULL (nothing, zero, void, nada) then there is no way the method gets executed. Then the runtime let you know this by throwing a NullPointerException.

Your reference is "pointing" to null, thus "Null -> Pointer".

The object lives in the VM memory space and the only way to access it is using this references. Take this example:

Some reference = new Some(); // Point to a new object of type Some()
Some otherReference = null; // Initiallly this points to NULL
reference.setId( 1 ); // Execute setId method, now private var id is 1
System.out.println( reference.getId() ); // Prints 1 to the console
otherReference = reference // Now they both point to the only object.
reference = null; // "reference" now point to null.
// But "otherReference" still point to the "real" object so this print 1 too...
System.out.println( otherReference.getId() );
// Guess what will happen
System.out.println( reference.getId() ); // :S Throws NullPointerException because "reference" is pointing to NULL remember...

This an important thing to know - when there are no more references to an object (in the example above when reference and otherReference both point to null) then the object is "unreachable". There is no way we can work with it, so this object is ready to be garbage collected, and at some point, the VM will free the memory used by this object and will allocate another.

operation on uninitialized object at instance level(not the class level) will lead to NullPointerException. operation need to be instance specific. if operation is at class level, saying calling a static method on uninitialized object then it will not throw NullPointerException exception. Even primitive wrapper class objects throws NullPointerException.
– Shailendra SinghJul 8 '16 at 15:20

1. NullPointerException is a RuntimeException, that means will appear when your program is running, you will not at compilation time.! :(, but most of the IDE help you to discover this. 2. Minimize the use of the keyword 'null' in assignment statements. :) Reference url:
– tomj0101Dec 4 '17 at 5:49

@tomj0101 I'm thoroughly unclear as to why you made that comment... But to your second point, a pattern before Optional was to return null. The keyword is fine. Knowing how to guard against it is critical. This offers one common occurrence of it and ways to mitigate it.
– MakotoDec 4 '17 at 5:54

NullPointerException is a run-time exception which is not recommended to catch it, but instead avoid it.
– ShomuMay 29 '18 at 10:14

@Shomu: At what point do I even suggest that it should be caught?
– MakotoMay 29 '18 at 13:25